FIG. 1 is a schematic cross sectional view showing the film structure of a typical example of known magnetic recording medium. In this recording medium, a first base layer 2 made of Ni--P, Al or the like is formed on a substrate 1 made of glass, Al or the like, to provide a base 11, on which are formed a second base layer 3, a single magnetic layer 4 (formed of CoCrPtB or CoCrPtTa, for example), and a protective layer 5 made principally of C. Further, a lubricant layer 6 consisting of a liquid lubricant is formed on the protective layer 5. To produce such a recording medium, the first base layer 2 formed from a Ni--P or Al film is formed by a wet film-forming process, such as electroless plating, or a dry process, such as sputtering in a vacuum, or vapor deposition, on a surface of the non-magnetic substrate 1 which is made of a glass material or Al alloy, for example, and is machined to achieve required degrees of parallelism, flatness and surface roughness, so as to provide the base 11. The base 11 is then processed again by mechanical texture machining and/or laser texture machining, to achieve desired degrees of flatness and surface roughness. The surface of the base 11 is then brought into a purified condition by cleaning, for example. Subsequently, the base 11 is heated to 50-300.degree. C. in a vacuum, and the second base layer 3 made of Cr and having a film thickness of about 50 nm, magnetic layer 4 made of CoCrPtTa whose major component is Co and having a film thickness of about 30 nm, and the protective layer 5 made principally of C and having a film thickness of about 10 nm are formed by dc sputtering on the surface of the base 11, while a dc bias voltage of about -200 V is being applied to the base. Then, the protective layer 5 is coated in the atmosphere with a fluorocarbon containing liquid lubricant which provides the lubricant layer 6 having a film thickness of 1 nm. In this manner, the magnetic recording medium is produced. The recording medium thus produced exhibits good mechanical characteristics, such as strength and dimensional accuracy, causing no problems in practical use, and also exhibits good magnetic characteristics. More specifically, the coercive force (Hc) is about 2000 Oe, and the product (Brt) of the residual magnetic flux density and the film thickness is about 150 G.mu.m. The slope (S*) of the magnetization curve in the vicinity of Hc is about 0.85, which is also favorable.
If the second base layer 3 of the above recording medium is divided into an NiAl layer as a first layer, and a Cr layer as a second layer, and the magnetic layer 4 is formed of CoCr.sub.10 Pt.sub.15 (at. %) entirely by rf sputtering, the resulting recording medium provides a high coercive force (Hc) of 3000 Oe or higher (refer to Li-Lien Lee et. al.: IEEE Trans. Magn., 31, 2728 (1995)).
The present invention is concerned with a technique for further improving the above-described magnetic recording medium.
To meet with a recent requirement for processing of a large amount of information whose volume and diversity are rapidly increased, the fixed magnetic disc device is strongly desired to provide a high recording density and a large capacity. To this end, the magnetic recording medium used in the magnetic disc device is desired to have a high linear recording density, reduced noise (N), and good electromagnetic conversion characteristics. To reduce the noise, magnetic particles of the magnetic layer need to have a relatively small particle size and a relatively high degree of magnetic isolation (refer to M. Takahashi, et al: EEE Trans. Magn., 31, 2833 (1995)), and the coercive force (Hc) needs to be increased to a certain extent in order to maintain a high linear recording density.
Further, there is a need to manufacture a large quantity of magnetic recording media so as to lower the cost per product, while assuring a high percentage of non-defectives (yield). To this end, it is desirable to employ a further simplified method for manufacturing the recording media.